1. Field of the Invention
The present invention relates to antenna and information processors, and more particularly to an antenna structure effective when applied to a mobile computer system such as a notebook-sized personal computer, etc.
2. Background Art
With the spread of the utilization of Internets, Intranets and other intra-office networks, much interest and a great deal of effort have recently been devoted to radio communication devices for personal computer (PC) users to connect to a telephone network or local area network (LAN). In addition, with practical use and generalization of the Bluetooth standard, radio communication devices with high utilization have positively been developed.
Antennas are very important in radio communication techniques along with transceivers. The antenna, as is well known, is a device to radiate and detect an electronic wave efficiently, and a wide variety of antennas are known. For instance, a vertical antenna (unipolar antenna) and a dipole antenna are known as linear antennae, and a loop antenna and a folded antenna are known as line antennae. A slot antenna (aperture antenna), a bipolar antenna, and a horn antenna are known as solid-state antennae. In applying these antennae to computers, or a multiplicity of devices meeting the Bluetooth standard, it is necessary to take into consideration a reduction in the device manufacturing cost, ease in assembling, convenience during operation, reliability, etc., as well as a reduction in a mounting space.
Because of this, aperture antennae (slit antennae) have been adopted in equipment, which is limited in mounting volume, such as notebook-sized personal computers, etc. The aperture antenna is constructed of a thin metal plate having a predetermined size of slit (aperture). It has excellent costs because machining is easy and is excellent in mounting as it can be mounted in a small volume. In addition, if it is given a certain degree of mechanical strength, it can be utilized as a component supporting member and therefore it can make a contribution to a reduction in the number of steps. Furthermore, an antenna element does not protrude as it does in the case of a linear antenna such as a dipole or unipolar antenna. Therefore, appearance can be enhanced from the viewpoint of design, and an unexpected accident due to catching, etc., of the protruding portion during operation can also be prevented.
However, it is difficult to perform impedance matching between an aperture antenna and a transmission line without employing any matching element. In general, the matching between an aperture antenna and a transmission line (cable) is done by a method of finding out the optimal driving point of the cable by experiment or simulation. This method is possible when a difference impedance with the cable is not large. Since, however, the radiation resistance of an ordinary aperture antenna is high such as 500 xcexa9 to 5000 xcexa9, proper impedance matching cannot be performed, unless an impedance matching circuit (element), such as a transformer, etc., is inserted between a cable and an antenna. Insertion of the impedance matching element results in an increase in the mounting volume, as well as a rise in costs resulting from an increase in the number of components and the number of manufacturing steps. Particularly, applications in products, which have limited mounting volume, such as a portable information terminal, etc., are not preferable.
Besides, it is fairly difficult to determine the optimal position of the aforementioned driving point. More specifically, as impedance is largely changed by a slight change in the driving point, there is a need to determine the driving point precisely. This makes the design of produces difficult, and also requires high machining accuracy, resulting in a rise in the manufacturing cost. Furthermore, an ordinary aperture antenna is narrow in band. Particularly, in the case where it is used with a frequency in the order of GHz, antenna characteristics are degraded by changes in machining dimensions. An enhancement in machining precision increases manufacturing costs, as described above. Even if the aperture antenna is manufactured with a high degree of machining precision, it cannot be used in a band offset from the designed wavelength and will be inferior in convenience.
In addition, in an ordinary aperture antenna, driving voltage (current) is applied between both ends of the aperture. Because of this, the electric potential across a metal plate with an aperture is not statically fixed like ground potential. This means that it tends to undergo the influence of the surrounding potentials, and can be the cause of unstable antenna characteristics.
On the other hand, it is considered that another antenna can be used instead of the aperture antenna. However, in the case of a dipole antenna, for example, it increases the number of components, and if it is mounted inside a device, it will increase a mounting volume. If it is projected from a device, the aforementioned disadvantages will arise. Other antenna elements also have similar disadvantages.
It is an object of the present invention to provide an aperture antenna that is capable of matching with a cable (transmission line) without employing any matching circuit (matching element). Another object is to provide an aperture antenna that is capable of allowing slight fluctuations in machining. Still another object is to provide an aperture antenna in which its operating band is broad. Yet a still another important object is to provide an aperture antenna, which is less liable to be affected by the influence of operating circumstances or surrounding electric fields.
An antenna according to the present invention is schematically constructed as follows:
A feature of the present invention includes an antenna having a first flat electric conductor with an aperture in which a second aperture length in a second direction perpendicular to a first direction is greater than a first aperture length in the first direction. A second electric conductor, spaced from the first electric conductor is disposed inside the aperture in practically the same plane with the first electric conductor. A non-balance type transmission line has its signal line connected to the second electric conductor and its ground line connected to the first electric conductor. The center-to-center distance, in the second direction, of a gap between the first and second electric conductors is one half an electrical length in the frequency band of an electronic wave signal radiated.
Various other objects, features, and attendant advantages of the present invention will become more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views.